Failure of Boiler Economiser

R-TECH Materials received two lengths of tube from a boiler economizer tube from a power station (see Figure 1). One of the samples had developed a leak. The area of rupture is shown in Figure 2. R-TECH Materials were requested to determine the cause of the rupture.

Figure 1. Economiser tube samples

Dye penetrant testing (DPI) was undertaken, but no defects were detected other than the perforation shown in Figure 2. A section was taken through the area of the perforation, and etched for metallographic preparation. This showed that the cracking initiated in the Heat Affected Zone (HAZ) of the butt weld (see Figure 3). There were no material defects associated with the cracking.

Sections taken away from the perforation showed the presence of numerous straight cracks all containing oxide (see Figures 4 and 5). These cracks are characteristic of thermal fatigue, and this was identified as the failure mechanism in this case.

Figure 2. Area of rupture

Figure 3. Section through area of rupture, etched

Figure 4. Cracking on inner surface of bend

Figure 5. Cracking on inner surface (etched) showing oxide